Vaccine antigens are frequently adsorbed to insoluble metal salts, such as aluminium hydroxide and aluminium phosphate, to provide an adjuvant effect. It is often necessary to analyse antigens after their adsorption, but the adsorption itself can make analysis difficult e.g. as described in reference 1 for hepatitis B surface antigen (HBsAg).
Rather than testing adsorbed antigens directly, many assays instead rely on a desorption treatment, followed by analysis of the released antigen. For example, to measure the proportion of adsorbed antigen it is known to test for unadsorbed antigen before and after a desorption treatment, with the difference being used to infer the amount of adsorption. In this type of assay, however, the requirement for a desorption step is cumbersome, and it also means that the antigen is not being analysed in the form in which it is actually supplied or used.
Direct in situ quantification of adsorbed antigens has been reported e.g. by ELISA in reference 2, or by near-infrared spectroscopy in reference 3. Reference 4 discloses a direct alhydrogel formulation immunoassay (“DAFIA”) which is designed to directly (i.e. without prior extraction), accurately, sensitively, and specifically determine the content, identity and integrity of an antigen while it remains adsorbed to an aluminium hydroxide adjuvant.
It is an object of the invention to provide further and improved methods for analysing vaccine antigens (or other components) which are adsorbed to insoluble metal salts.